The present invention relates in general to real-time filters and, more particularly, to generation of zeroes for a real-time tracking filter circuit.
It is known in the art that hard disk drives and sampled data systems require optimized signal-to-noise ratios and demand matched transfer functions in order to achieve error rates of less than 10.sup.-11. In order to achieve the desired level of signal quality, such data communication systems incorporate peak detection techniques utilizing extremely accurate real-time tracking filters. The poles of these filters are typically located at frequencies up to 100 megahertz. Equally important are accurately placed associated zeros that influence the peaking characteristic of the filter.
In the past, the filtering function has been implemented by providing a differential input signal to a first summing block. The output of the first summing block is passed to a first transconductance block which in turn drives a second summing block. The second summing block further drives a second transconductance block that drives the first nodes of each of two integrating capacitors. Typically, a parallel path is established, with the input signal applied to a differential amplifying block whose outputs drive the second nodes of either or both the two integrating capacitors, respectively, thus completing the low-pass filtering response loop.
It is well known in the art that miniaturization of electronic circuits such as the aforedescribed low-pass filter circuit is highly desirable, and furthermore, it is known that monolithic integrated circuit structures provide economical and efficient means for miniaturization. However, a monolithic implementation of the low-pass filter circuit presents new obstacles that must be overcome in order to achieve the level of performance required in real-time data communication systems.
At each integration node located at the outputs of the transconductance blocks there is parasitic stray capacitance. The parasitic stray capacitance creates an attenuation effect that modifies the predicted pole-zero relationship of the low-pass filter. Attempts have been made to compensate the pole-zero response by adding a nominal parasitic capacitance. Unfortunately, the actual capacitance added is subject to processing variation and therefore difficult to control. Moreover, the additional capacitance requires additional drive from the amplifying blocks to maintain the desired filter response.
Hence, a need exists for a low-pass filter circuit with a response that can be effectively and predictably peaked, and furthermore, is insensitive to the adverse effects of stray parasitic capacitance.